1. Field of Invention
The present invention relates generally to a timepiece. More particularly, the invention relates to a structure for fastening the case, the bezel, the back cover and other external parts, and the movement.
2. Description of Related Art
In conventional timepieces, and particularly timepieces that are highly water resistant, the bezel 94 is usually press fit to the top edge of the case 92 with intervening annular plastic packing PK as shown in FIG. 10. The plastic packing used around the bezel is harder than rubber packing and is used as a fastening member for fastening bezel to the case in addition to being used to provide water resistance. As also shown in FIG. 10, a common method of fastening the back cover of a water resistant timepiece is to thread the outside edge of the back cover 95, which effectively becomes a screw, and screw the back cover 95 into the inside edge of the case 92.
The movement 91 is fastened to the case 92 with screws into fastening parts formed on the inside circumference part of the case 92 and to a cylindrical spacer 96 at approximately two places on the outside of the movement 91.
Because the prescribed removal force of the plastic packing between the bezel and the case member is assured in a structure that uses plastic packing around the bezel by increasing the thickness of the bezel in the thickness direction of the timepiece, the bezel and case necessarily become thicker through the thickness direction of the timepiece in order to ensure the prescribed fastener strength.
In addition, if the back cover screws to the case, the back cover and case member require a prescribed material thickness and size in the thickness direction of the timepiece in order to form threads on the back cover and case member, and the back cover and case necessarily become thick and large.
A problem with the conventional method of fastening the bezel, movement, and back cover, therefore, is that because the full circumference of the bezel, the full circumference of the movement, and the full circumference of the back cover are used for fastening, these fastening structures cannot be neatly arranged in the same space, and the case member becomes bulkier than the thickness and diameter required to achieve the required fastening strength. More specifically, space is needed around the outside of the timepiece for fastening by means of the plastic packing, screwing in the back cover, and fastening the movement with screws, the fasteners for the movement must be located to not interfere with the plastic packing that secures the bezel, and the fastener of the back cover must be located to not interfere with the fasteners of the movement, and the diameter of the case thus increases.
As a result of using different fastening means for the bezel, back cover, and movement, the bezel, back cover, and movement become thicker, and the timepiece becomes larger and heavier. Note that the problem is that the fastening means for the bezel, the back cover, and the movement are designed separately, and an increase in the timepiece diameter is unavoidable even when the back cover is fastened by a screw as taught in Japanese Unexamined Patent Appl. Pub. JP-A-2000-205222, for example.